Pull the power meter off. Inside, the neutral I am told, should run straight through to the panel. It should be in one peice, not clamped together. I have personally witnessed the corrosion that builds up under the clamp, but when cleaned to a shine and clamped again the hum went away.

I have also seen corroded connections at the weatherhead cause hum problems. Especially in older homes with aerial power, these connections were simply taped and after years in the elements the corrosion built up. Your local power company should be responsible for everything up to and including the meter, so have them check these points if it comes to that.